The present invention pertains to a plasma atomic emission spectroscope and particularly to a power control circuit therefore.
Inductively coupled plasma atomic emission spectroscopy analysis is a technique for determining major, minor and trace elemental constituents of liquid samples. This technique is based upon a generation of an inductively heated plasma utilizing RF energy. Liquid samples are converted into an aerosol utilizing a nebulizer and injected into a plasma. As a sample enters the plasma, it undergoes desolvation, volatilization, atomization, excitation and finally emits photons characteristic of wavelengths of elements present in the sample. The intensity of the emission at the characteristic of the wavelengths is used to determine the concentration of a element present in the sample while the wavelengths themselves determine what elements are present. The plasma is typically supported by an argon gas, but other gases may also be employed. The most commonly used frequency for maintaining the plasma has been 27.12 Mhz, however, recently other frequencies including 40.68 Mhz have been employed.
The equipment available to date for activating the plasma torch for such spectroscopes includes single frequency excitation systems, which although working well with certain samples, does not provide the flexibility of providing optimum plasma excitation for different samples. Thus for example, alkali elements such as sodium, lithium and potassium can be adequately excited at a frequency of 27.12 Mhz providing a plasma temperature of 6000.degree.-8000.degree. K. while metal, such as aluminum, tungsten, molybdenam and iron are better excited at a higher temperature plasma available with an excitation frequency of 40.68 Mhz which provides a plasma temperature in the range of 6,000.degree.-10,000.degree. K. The dual frequency system also allows flexibility in analyzing samples dissolved in various solvents such as water or organic solvents. The frequency available for use with such equipment is limited by FCC regulations and therefore, not a great deal of flexibility is available for experimentation on excitation frequencies. In the past, commercially available instruments, such as manufactured by Henry Radio, model no. Henry-2000-D-40, have provided a single excitation frequency. Some attempts have been made to provide multiple frequency operation by physically changing tank circuit components which is both time consuming, expensive and somewhat difficult inasmuch as the output of the excitation or power control circuit terminates into an induction coil for the plasma which remains the same, for the multiple frequencies. Thus it has been difficult to provide a single instrument with dual frequency characteristics for the optimum plasma excitation for the different elements and different solvents.